There isnt an easy answer to your question. It depends on a LOT of factors. Size of targets, minerals and trash, your settings... and you. If you know what you are looking for about 9" on coins with an occassional deeper target.
The stock coil for the dfx is one of the best for the unit.
Like Shrecky said. Kick up the a.c and pre amp to max if stable. Set the threshold around 6. I've hit dimes at 9 inches cranked and in good soils.
I now use the 1400 all the time. But the stock performes very well. I like the coverage it gives. I've got large cents down 10 to 12 inches with the 1400 and the unit cranked up.
Lot of unit to learn. First get familar with all the tones and vdi numbers. Dig alot of targets from foil to silver. You'll get the hang of it.
believe it or not my MXT found a silver shoe button in my front yard that was at least 12 inches deep, with the stock 9.5 coil. but the average "deep" things iv've been finding are aprox 8-9 in. deep.
In perfect condidtions, my 6000 Di Pro has found dimes at 8". However, most conditions require tweaking of the sensitivity and my usual finds are less than 7" with the 9.5.
Is there a reason you are asking? Is your stock coil not working right? Mine suddenly would not go deeper than 4.5 White's did replace it...just received new one is air testing deeper.... Snowing now...hopefully can ground test tomorrow...
I will be happy if my replacement coil does that(8 inches)...air testing tonight...strapping the new coil on now...cannot find the basic hardware...but I do have the bracket so the coil does not move...which will be fine if this ice/snow melts for tomorrow...
I pulled a dime last week at 7 inches with the AC at 68 and the Pre amp at 3. I got a buffalo last year at about 9 inches, only it read as nickle, quarter. The depth does that though. Pretty good coil...